This invention relates to a process of initiating the polymerization of an anionically polymerizable monomer to prepare a polymer having a low polydispersity index and high tensile strength. Polymers of this type are useful as adhesives and elastomers.
Much effort has been directed to the preparation of polydienes with precisely-controlled molecular weight, functionality, and microstructure (that is, 1,4 addition). For example, Tung et al. (U.S. Pat. No. 4,196,154, incorporated herein by reference) discloses the polymerization of a conjugated diene to give a high degree of 1,4 addition. This polymerization reaction is initiated by 1,3-phenylenebis(3-methyl-l-phenylpentylidene)bis(lithium), a liquid hydrocarbon soluble difunctional organolithium initiator (DFI), shown below: ##STR1##
Tung et al. discloses that "for uniformity of the product and maximum control, as well as ease of handling, it is desirable that an initiator be soluble in the polymerization system, rather than merely dispersible as a particulate material."
Lo et al. (U.S. Pat. No. 4,960,842, incorporated herein by reference) discloses a method of lowering the polydispersity of a polymer by initiating the polymerization of at least one anionically polymerizable monomer using the specific DFI described in Tung et al.; and carrying out the polymerization in the presence of an organic diamine or triamine. Though the polymers described by Tung et al. and Lo et al. exhibit the desired tensile strength and polydispersity, commercial viability is impaired because the process requires an expensive DFI.
Attempts to prepare useful polymers with a less expensive DFI, such as 1,3-bis(1-lithio-1,3-dimethylpentyl)benzene, shown below, have not been successful, presumably due to the difficulty in preparing a pure DFI. ##STR2##
For example, attempts to prepare this DFI from m-diisopropenylbenzene and sec-butyllithium in the presence of low levels of triethylamine does not give a pure diadduct, but rather "a mixture of bi- and polyfunctional material, together with unreacted sec-butyllithium" (see Cameron et al., Polymer, 20, 1129, 1132 (1979), incorporated herein by reference). Cameron et al. concludes that "the use of this system as a bifunctional initiator must be viewed with suspicion."
It is therefore desirable to prepare polymers, and more particularly triblock copolymers, having a polydispersity of less than 1.4 and tensile strength of greater than 1500 psi, using an inexpensive, high purity DFI that is rendered soluble in a nonpolar solvent.